1. Field of the Invention
The invention relates to a multi-layer circuit board, more particularly to a multi-layer circuit board which can achieve impedance matching to result in reduced high speed signal reflection and reduced electromagnetic interference.
2. Description of the Related Art
According to industrial standards, multi-layer circuit boards with eight wiring layers generally include the 1.2 mm type and the 1.6 mm type. FIG. 1 shows a conventional multi-layer circuit board with a thickness of about 1.2 mm, which includes: first, second, third, fourth, fifth, sixth and seventh insulating substrates (A1), (A2), (A3), (A4), (A5), (A6), (A7) disposed sequentially one above the other; a first signal wiring layer (S1) disposed on one side of the first insulating substrate (A1) opposite to the second insulating substrate (A2); a first ground wiring layer (GND1) disposed between the first and second insulating substrates (A1) (A2); a second signal wiring layer (S2) disposed between the second and third insulating substrates (A2), (A3); a third signal wiring layer (S3) disposed between the third and fourth insulating substrates (A3), (A4); a power wiring layer (POWER) disposed between the fourth and fifth insulating substrates (A4), (A5); a fourth signal wiring layer (S4) disposed between the fifth and sixth insulating substrates (A5), (A6); a second ground wiring layer (GND2) disposed between the sixth and seventh insulating substrates (A6), (A7); and a fifth signal wiring layer (S5) disposed on one side of the seventh insulating substrate (A7) opposite to the sixth insulating substrate (A6) Each of the first, third, fifth and seventh insulating substrates (A1), (A3), (A5), (A7) is made from a polyester prepreg. Each of the second, fourth and sixth insulating substrates (A2) (A4), (A6) is made from a fibrous core material that contains paper or glass fibers.
In addition, each of the first and seventh insulating substrates (A1), (A7) has a thickness (H4) of about 2.5 mil. Each of the second and sixth insulating substrates (A2), (A6) has a thickness (H3) of about 8 mil. Each of the third and fifth insulating substrates (A3), (A5) has a thickness (H2) of about 5 mil. The fourth insulating substrate (A4) has a thickness (H1) of about 8 mil. The first signal wiring layer (S1) has a first resistance (Rs1) with respect to the first ground wiring layer (GND1). The second signal wiring layer (S2) has a second resistance (Rs2) with respect to the first ground wiring layer (GND1) and the power wiring layer (POWER). The third signal wiring layer (S3) has a third resistance (Rs3) with respect to the first ground wiring layer (GND1) and the power wiring layer (POWER). The fourth signal wiring layer (S4) has a fourth resistance (Rs4) with respect to the second ground wiring layer (GND2) and the power wiring layer (POWER). The fifth signal wiring layer (S5) has a fifth resistance (Rs5) with respect to the second ground wiring layer (GND2). The first and fifth resistances (Rs1), (Rs5) are about 44 ohms. The second, third and fourth resistances (Rs2), (Rs3), (Rs4) are about 51 ohms.
FIG. 2 shows a conventional multi-layer circuit board having a thickness of about 1.6 mm. The 1.6 mm circuit board differs from the above-described 1.2 mm circuit board in that H4 is equal to 9.5 mil, instead of 2.5 mil. Hence, the first and fifth resistances (Rs1), (Rs5) are about 76.4 ohms.
Due to their construction, the conventional multi-layer circuit boards shown in FIGS. 1 and 2 have the following disadvantages:
1. Serious High Speed Signal Reflection
According to the standard theoretical values determined by Intel, the resistance between two adjacent wiring layers for a circuit board during high speed signal transmission is preferably within the range of 55xc2x110% xcexa9, i.e., between 49.5 xcexa9 and 60.5xcexa9. However, the first and fifth resistances (Rs1), (Rs5), being about 44 ohms, of the aforementioned conventional 1.2 mm circuit board fall outside the preferred range recommended by Intel, and there is additionally a difference of 7 ohms between the value of the first and fifth resistances (Rs1), (Rs5) and that of the second, third and fourth resistances (Rs2), (Rs3), (Rs4) Such a difference will result in an impedance mismatch. Thus, when a high speed signal is being transmitted through the conventional 1.2 mm circuit board and passes from the first or fifth wiring layer (S1) or (S5) to the second, third or fourth wiring layer (S2) or (S3) or (S4), reflection of the signal will result, thereby adversely affecting signal transmission. Likewise, the value of the first and fifth resistances (Rs1), (Rs5) of the aforementioned conventional 1.6 mm circuit board falls outside the theoretical range, and there is a large difference present between the value of the first and fifth resistances (Rs1), (Rs5) and that of the second, third and fourth resistances (Rs2), (Rs3), (Rs4). The reflection index of high-speed signals for the 1.2 mm circuit board can be calculated as follows:   ρ  =                    Zl        -        Zo                    Zl        +        Zo              =                            Rs1          -          Rs2                          Rs1          +          Rs2                    =      0.07      
The reflection index for the 1.6 mm circuit board can be calculated in a similar manner and is found to be even higher, being 0.2. The signal reflection is therefore very serious, which results in considerable distortion of the waveform and poor signal quality.
(2)Weakened magnetic flux counteraction
As reflection of high speed signals will generate standing waves, which will increase electromagnetic radiation of the high speed signals, the magnetic flux counteraction of the circuit board is weakened, thereby resulting in excessively high electromagnetic interference.
Therefore, the main object of the present invention is to provide a multi-layer circuit board which can achieve impedance matching to result in reduced high speed signal reflection and reduced electromagnetic interference.
Accordingly, a multi-layer. circuit board according to the present invention includes: first, second, third, fourth, fifth, sixth and seventh insulating substrates disposed sequentially one above the other; a first signal wiring layer disposed on one side of the first insulating substrate opposite to the second insulating substrate; a first ground wiring layer disposed between the first and second insulating substrates; a second signal wiring layer disposed between the second and third insulating substrates; a third signal wiring layer disposed between the third and fourth insulating substrates; a power wiring layer disposed between the fourth and fifth insulating substrates; a fourth signal wiring layer disposed between the fifth and sixth insulating substrates; a second ground wiring layer disposed between the sixth and seventh insulating substrates; and a fifth signal wiring layer disposed on one side of the seventh insulating substrate opposite to the sixth insulating substrate. Each of the first and seventh insulating substrates has a thickness ranging from 2.5 to 6.5 mil. Each of the second, fourth and sixth insulating substrates has a thickness ranging from 3 to 9 mil. Each of the third and fifth insulating substrates has a thickness ranging from 3 to 23 mil. The first signal wiring layer has a first resistance with respect to the first ground wiring layer. The second signal wiring layer has a second resistance with respect to the first ground wiring layer and the power wiring layer. The third signal wiring layer has a third resistance with respect to the first ground wiring layer and the power wiring layer. The fourth signal wiring layer has a fourth resistance with respect to the second ground wiring layer and the power wiring layer. The fifth signal wiring layer has a fifth resistance with respect to the second ground wiring layer. The first, second, third, fourth and fifth resistances are within the range of 49.5 to 60.5 ohms.